This application relates generally to magnetic disc drives and more particularly to a dummy magnet in a voice coil motor assembly for reducing the pitch and roll torque constants of the voice coil motor.
As shown in FIG. 1, typical modem hard disc drives 100 comprise one or more rigid discs 108 that are coated with a magnetizable medium and mounted on the hub 106 of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks by an array of transducers (xe2x80x9cheadsxe2x80x9d) 118 mounted to a controllably positionable radial actuator assembly 110 for movement relative to the discs.
Typically, such radial actuators 108 employ a voice coil motor 124 to position the heads 118 with respect to the disc surfaces. The heads 118 are mounted via flexures 116 at the ends of one or more actuator arms 114 which project outward from an actuator body 120. The actuator body pivots about a cartridge bearing assembly 112 mounted to the disc drive base plate 102 at a position closely adjacent to the outer extreme of the discs so that the heads move in a plane parallel with the surfaces of the discs.
The voice coil motor 124 includes a coil 126 mounted radially outward from the cartridge bearing assembly 112, the coil being immersed in the magnetic field of a magnetic circuit of the voice coil motor. The magnetic circuit comprises one or more permanent magnet pairs 128 and magnetically permeable pole pieces 140. When current is passed through the coil, an electromagnetic field is established which interacts with the magnetic field of the magnetic circuit such that the coil, as well as the actuator(s), experience rotational forces or torques about the axis of the cartridge bearing assembly 112.
As shown in FIG. 2, there are typically three principal torques experienced by the actuator assembly 110 and voice coil motor 124 as a result of the application of current to the coil. The first torque, often called the main torque, causes the coil 126 and the actuator arm(s) 114 to rotate about a central axis 250 (z-axis) of the cartridge bearing assembly 112, as shown by arrow 252. The second torque, referred to as the roll torque, causes the coil 126 and the actuator arms(s) 114 to rotate or twist about an axis 254 (x-axis) of the cartridge bearing assembly 112, as shown by arrow 256. The third torque, referred to as pitch torque, causes the coil 126 and the actuator arm(s) 114 to rotate or twist about an axis 258 (y-axis) of the cartridge bearing assembly 112, as shown by arrow 260. As is known, the main torque is the primary means by which the voice coil, and thus the head, are moved radially across the disc(s) 108. Stated another way, the main torque is a desired force which causes the actuator(s) and head(s) to move in a plane parallel with the disc(s) 108. In contrast, both the roll and the pitch torques cause motions in the actuator arms(s), head(s), and coil 126 which are not parallel to the plane of the disc(s). As such, the roll and the pitch torques adversely affect the head slider""s ability to maintain optimal flying height and to stay parallel to the disc(s) over the data tracks, thereby interfering with the read/write operation of the head in the disc drive.
Another problem associated with excessive pitch and roll torques in a disc drive relates to the introduction of torque induced noise into the disc drive""s servo positioning system. A closed loop digital servo system such as disclosed in U.S. Pat. No. 5,262,907, issued Nov. 16, 1993 to Duffy et al., assigned to the assignee of the present invention, is typically utilized to maintain the position of the heads with respect to the tracks. Such a servo system obtains head position information from servo blocks written to the tracks during disc drive manufacturing to maintain a selected head over an associated track during a track following mode of operation. A seek mode of operation, which comprises the initial acceleration of a head away from an initial track and the subsequent deceleration of the head towards a destination track, is also controlled by the servo system. Such seek operations are typically velocity controlled, in that the velocity of the head is repetitively measured and compared to a velocity profile, with the current applied to the coil being generally proportional to the difference between the actual and profile velocities as the head is moved toward the destination track.
A continuing trend in the industry is to provide disc drives with ever increasing data storage and transfer capabilities, which in turn has led to efforts to minimize the overall time required to perform a disc drive seek operation. A typical seek operation includes an initial overhead time during which the disc drive services its own internal operations, a seek time during which the head is moved to and settled on the destination track, and a latency time during which the drive waits until a particular sector on the destination track reaches the head as the discs rotate relative to the heads.
Seek times have typically been minimized through the application of relatively large amounts of current to the coil during the acceleration and deceleration phases of a seek operation. One way of reducing seek time is to increase the relative amount of current to the coil, thus causing an increase in the main torque and a resulting increase rotation of the coil, actuator, and head(s) about the cartridge bearing assembly. However, applying an increased current to the coil also increases the associated and unwanted roll and pitch torques.
Another drawback associated with the application of relatively large amounts of current to the coil during the acceleration and deceleration phases of a seek operation is the occurrence of mechanical vibrations in the voice coil motor, and hence the disc drive itself, as a result of the unwanted roll and pitch torques. These vibrations may induce noise into the servo control loop of the disc drive, thus making accurate track following difficult. As will be understood, the negative affects of vibrationally induced noise in the servo system are compounded as the track density or tracks per inch (TPI) of the disc drive is increased. As the general trend in the disc drive industry is to produce disc drives having ever increasing TPI, it is imperative that new methods and techniques are developed to address vibrationally induced servo system noise.
Along with the general trend in the industry to provide disc drives having greater TPI, there is also a trend to reduce the level of acoustic emissions generated by disc drives. A primary source of acoustical emissions from a disc drive is the amplification of the aforementioned vibrations of the voice coil motor by the top cover and by the base of the disc drive. These vibrations occurring in the voice coil motor may be transmitted to the top cover and/or the disc drive base either as sympathetic vibrations or as direct transmissions.
Torques occurring in electric motors such as the voice coil motor are typically defined in terms of the ratio of the output torque of a motor to its input current. This ratio, referred to as the torque constant of the motor, is generally expressed in units of force times length divided by current, or newton meters per ampere (Nm/A). As described above, the voice coil motor normally experiences three primary torques as a result of the application of current to the coil. The response of the actuator and the voice coil motor to the application of current to the coil may, therefore, be expressed in terms of three torque constants: the main torque constant; the roll torque constant; and the pitch torque constant.
Accordingly, there is a need for a voice coil motor assembly that has reduced roll and pitch torque constants but does not significantly increase the manufacturing costs by adding costly parts and/or manufacturing steps.
Against this backdrop the present invention has been developed. One embodiment of the present invention relates to a data storage device voice coil motor which utilizes a single permanent magnet and a dummy magnet in a magnetic circuit to reduce pitch and roll torques in the voice coil motor. The voice coil motor includes a first pole piece having an inner surface and a second pole piece having an inner surface. The second pole piece is connected to the first pole piece in a manner such that the inner surface of the second pole piece is held in spaced relation with the inner surface of the first pole piece and such that a gap is defined therebetween. In this arrangement, the second pole piece includes a dummy magnet which extends from the inner surface of the second pole piece into the gap. The permanent magnet is positioned within the gap on the inner surface of the first pole piece and a voice coil is positioned in the gap between the dummy magnet and the permanent magnet. The first pole piece, the second pole piece, the dummy magnet, and the permanent magnet form a magnetic circuit having lines of magnetic flux which flow in a substantially perpendicular manner to the surfaces of the dummy and permanent magnets through the gap, thereby reducing the pitch and roll torque constants of the voice coil motor, thus allowing a data storage device employing the voice coil motor to maintain optimal flying height of read/write transducers, to reduce vibrationally induced servo system noise, and to reduce acoustical emissions from the data storage device.
Another embodiment of the present invention relates to a method for reducing roll and pitch torque constants in a voice coil motor in a disc drive, wherein the disc drive includes a base plate, and wherein the voice coil motor includes a first pole piece, a second pole piece, a permanent magnet, and a voice coil. The method includes the steps of providing a stamped dummy magnet in the second pole piece, positioning the second pole piece in spaced relation to the first pole piece such that a gap is defined therebetween, positioning the permanent magnet on the first pole piece within the gap, and positioning the voice coil within the gap between the permanent magnet and the dummy magnet. As with the first embodiment of the present invention, the second embodiment of the present invention forms a magnetic circuit having lines of magnetic flux which flow substantially perpendicular to the surfaces of the dummy and permanent magnets through the gap, thereby reducing the pitch and roll torque constants of the voice coil motor.
These and various other features, as well as advantages which characterize the present invention, will be apparent from a reading of the following detailed description and a review of the associated drawings.